The increasing demand for electrical power has forced electrical utilities to bum increasing quantities of fossil fuels such as coal and oil. However, electric utilities also face increasing environmental standards imposed upon their operations by state and federal regulatory agencies that mandate reduced particulate and acid generating smoke stack emissions. To reduce acid generating emissions, electrical utilities have turned to burning low-sulfur coal in their boilers to generate the steam necessary for electric power generation. To reduce the particulate emissions, electric utilities generally use a flue gas treatment system to remove a majority of the particulate matter in the gas effluent passing out of the smoke stack. Such flue gas treatment systems typically comprise an electrostatic device such as an electrostatic precipitator or a fabric filter baghouse to remove the particulate. Such devices may also provide a source of conditioning agent to the flue gas to enhance the effectiveness of the precipitator or filter in removing the particulate.
The efficiency of an electrostatic precipitator in removing particulate matter from the boiler flue gas is partially dependent upon the electrical resistivity of the entrained particulate matter in the boiler flue gas. The entrained particulate matter expelled from a boiler fired with low-sulfur coal, i.e., coal having less than 1 percent sulfur, has been found to have a resistivity of approximately 10.sup.13 ohms/cm. It has been determined that the most efficient removal of particulate matter by electrostatic precipitation occurs when the particulate matter resistivity is approximately 10.sup.10 ohms/cm. Therefore, to obtain more effective use of an electrostatic precipitator, the resistivity of the entrained particulate matter from low-sulfur content coal must be reduced. Electrical utilities have long used conditioning agents introduced into the flue gas flow upstream of the electrostatic precipitator to reduce the resistivity of the entrained particles. Various chemicals, such as water, anhydrous ammonia, sulfuric acid, sulfur trioxide, phosphoric acid and various ammonia-bearing solutions have been used as conditioning agents.
In systems using sulfur trioxide as the conditioning agent, the sulfur trioxide is typically generated by combusting elemental sulfur in a sulfur furnace to generate sulfur dioxide. The sulfur dioxide is then passed through a catalytic converter which converts the sulfur dioxide to sulfur trioxide. A flue gas conditioning system of the type using sulfur trioxide as the conditioning agent is described in U.S. Pat. No. 5,032,154 to Robert A. Wright for a Flue Gas Conditioning System and assigned to Wilhelm Environmental Technologies, Inc., the assignee of this application. The disclosure of U.S. Pat. No. 5,032,154 is incorporated by reference.
Prior art systems have used the flue gas itself as the source of sulfur dioxide for conversion to sulfur trioxide. Flue gas generated when low sulfur coal is burned contains approximately 400 ppm to 1200 ppm of sulfur dioxide. In U.S. Pat. No. 3,581,463, a portion of the flue gas is withdrawn from the flue, electrostatically cleaned to remove particulate, then passed through a catalytic converter to generate sulfur trioxide. The sulfur trioxide is then injected back into the flue to condition the flue gas. U.S. Pat. No. 5,011,516 discloses such a system which eliminates the need to clean the withdrawn sulfur trioxide before passing it through the catalytic converter. U.S. Pat. No. 5,240,470, owned by the assignee of the invention described in this application, discloses using moving the catalytic converter into the flue duct where the flue gas flows through the catalyst and converts the SO.sub.2 contained in the flue gas to SO.sup.3 when conditioning agent is needed.
As is known, flue gas should not be over conditioned by injecting too much SO.sub.3 into it. If flue gas is over conditioned, all the SO.sub.3 does not interact with the flue gas and the excess SO.sub.3 is emitted from the stack, which is undesirable. U.S. Pat. No. 5,011,516 discloses regulating the amount volume of the flue gas withdrawn for treatment with the catalyst, i.e., converted to SO.sub.3. U.S. Pat. No. 5,240,470 discloses moving a catalytic converter into an operative position in the flue gas stream when conditioning agent is needed and to an inoperative position out of the flue gas stream when conditioning agent is not needed.
The problem with the above described techniques is that they require mechanical arrangements for controlling the amount of flue gas withdrawn from the flue, such as dampers or valves, or moving the catalytic converter into and out of the flue. Moreover, such control devices are less than precise.
It is an object of this invention to provide a flue gas conditioning system wherein native SO.sub.2 is catalytically converted to SO.sub.3 which is used to condition the flue gas and the amount of SO.sub.3 produced is more precisely controlled than prior art systems by varying the temperature setpoint at the inlet of the catalytic converter.